1. Field of the Invention
The present invention generally relates to a laser scanning unit for an image forming apparatus and more specifically, to a multi-beam laser scanning unit used in a color image forming apparatus.
2. Description of the Related Art
A laser scanning unit forms an electrostatic latent image on the surface of a photoconductive device, by scanning a laser beam to the photoconductive device. FIG. 1 illustrates a conventional laser scanning unit 10.
As shown in FIG. 1, the conventional laser scanning unit 10 includes a laser diode 11 to emit a laser beam, a collimating lens 12 to make the emitted laser beam parallel with regard to an optical axis, and a cylindrical lens 13 to change the parallel beam into a horizontal linear beam. The laser scanning unit 10 further includes a polygonal mirror 14 to reflect the horizontal linear beam within predetermined angles at a constant rate. The laser scanning unit 10 also has an Fθ lens 15 having a given refractive index with respect to another optical axis, which focuses the beam on a scanning surface by refracting the beam reflected at the constant rate from the polygonal rotating mirror 14 in a main scanning direction and adjusting for any aberration. The laser scanning unit 10 also includes a reflection mirror 16 to reflect the laser beam from the Fθ lens 15 in a predetermined direction, thereby making the beam be incident on the surface of a photoconductive drum 0, i.e., the scanning surface.
Meanwhile, the conventional image forming apparatus is provided with a plurality of the photoconductive drums O1, O2, O3, O4 to which developers with different colors adhere for color printing. Such an image forming apparatus includes a multi-beam laser scanning unit with a plurality of laser light sources in order to form a latent image on the respective photoconductive drum O1, O2, O3, O4. In FIG. 2, the conventional multi-beam laser scanning unit 100 is shown.
As shown in FIG. 2, the conventional multi-beam laser scanning unit 100 includes a laser light source unit 110 to emit laser beams corresponding to respective photoconductive drums O1, O2, O3, O4, a cylindrical lens 120 to make a plurality of parallel beams emitted from the laser light source unit 110 into horizontal linear beams, and a polygonal rotating mirror 130 to reflect the horizontal linear beam within given angles, and rotate at a constant rate. The laser light source unit 110 includes an Fθ lens 140 having a given refractive index with respect to an optical axis and to refract the beam emitted from the polygonal rotating mirror 130 in a main scanning direction, and a plurality of mirror units 150 to reflect the beams from the Fθ lens 140 to be incident on the surfaces of the respective photoconductive drums O1, O2, O3, O4.
In the laser light source unit 110, as shown in FIG. 3, arranged adjacent to each other are a plurality of laser diodes 111, 112, 113, 114 to emit laser beams and a plurality of corresponding collimating lens units 115, 116, 117, 118. The respective collimating lens units 115, 116, 117, 118 have collimating lenses 115a, 116a, 117a, 118a and slits 115b, 116b, 117b, 118b. 
In the conventional multi-beam laser scanning unit 100, however, the collimating lens units 115, 116, 117, 118 corresponding to the laser diodes 111, 112, 113, 114 are arranged in the vertical direction so that the height of the light source unit 110 increases. Moreover, as the gap between optical axes of laser beams L emitted from each of the laser diodes 111, 112, 113, 114 is broad, the height of parts arranged in the path of the laser beams L increases. As a result, the height H1 of the laser scanning unit 100 is also increased, thereby increasing the size of the image forming apparatus. Further, a motor 131 with a large output is necessary due to the increased height, which results in increased manufacturing costs.